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230

Eccentrics,

slipped on tp the shaft lengthwise. But there are many cases
where it is necessary that the sheave should be in halves for this
purpose, and the machining would-be then performed in a very
similar manner to the eccentric straps to be described, namely, by
bolting together the halves before turning. The drawing of the
sheave is shewn at A. Lay the casting level on the marking-table,
as at A!, and scribe the various thicknesses; span the hole, as at B,
and strike a circle for its diameter. Grip in the dog-chuck, as at c?
bore, and surface the projecting boss and the face of the sheave,
marking the diameter of the boss in the lathe. As the sheave
has to be chucked eccentrically, the face plate must, of course, be
balanced. Next reverse, and turn the opposite face of the
sheave, this time chucking centrally, as at D, and setting the
already turned boss close to the face plate. Lastly, the outer
circle is struck out, as at E, by re-bridging the centre, and marking
the exact eccentricity on the centre line at x, and the work is
then bolted to the face plate, as shewn at F, each portion of the
rim being measured in position, and carefully turned exactly to
dimension, because it must be a correct ' working fit' with the
strap. The key-way may be slotted out. (See Apf. /,/. 751.)

The Straps (drawn at a, Fig. 244) are first marked off, as at
AU and B! (Fig. 245), with the proper allowance for machining the
feet, and the two are then bolted down together to the planing
table, as at E^ The bolt holes are next scribed and squared, as
at DU the casting lying level on its side, and these are drilled, as at
Et. The thickness of the feet for the front strap being lined at
F!, and the stop for the bolt-head at H, these are cut out, the
first with pin drill, as at G (known as 'knifing' or lface-arboring'),
and the second with chisel and file. Now place face to face in
the vice, and broach the bolt-holes right through; then, having
turned the bolts to a good fit, fasten both straps together. Lay
the bolted straps level, as a,t j, scribe the width, and grip in a
dogchuck, as at K, to face both sides, setting carefully for each.
Leaving the work in the chuck, examine the outer rim for
centrality (for this cannot afterwards be turned), and mark off
the inner circle for boring, measuring with callipers and rule as
the work proceeds. Remove from the chuck, and scribe the
remaining sttrfaces—b c d, as at M, measuring from the turned